When cargo is being handled by a hydraulic loading crane and, while being lowered at full speed, the crane arm is suddenly stopped, the added dynamic load on the crane arm, caused by its suddenly arrested motion, becomes considerable. Such added load must be reckoned with when the durability and strength of the loading crane is determined. One has attempted to reduce the added dynamic load and to make better use of the available loading capacity of the crane by connecting a gas accumulator to the load-supporting hydraulic cylinder. The gas accumulator is set to accommodate a pressure which is somewhat higher than that normally prevailing in the associated hydraulic system. The pressure increase generated by the added dynamic load in the cylinders and in the hydraulic system of the loading crane then forces into the accumulator the amount of hydraulic fluid that is required to retard the movements of the arm system for a length of time that is sufficient to reduce the added dynamic load to an acceptable level.
The pressure of the fluid thus forced into the accumulator is of a magnitude above the static pressure prevailing in the hydraulic system of the crane. Consequently, this amount of fluid returns to the hydraulic system when the retardation movement has ceased. This means that there is a temporary increase of the amount of fluid enclosed in the cylinder, and this increased amount of fluid forces the piston rod of the cylinder somewhat outwards, imparting a jerky return movement to the crane arm. These movements are, however, very small and as a rule they are negligible under normal conditions.
When working with loading cranes under special environmental conditions, such as is the case in loading operations with the aid of vessel-mounted loading cranes when loads are transferred from one boat to the other under heavy sea conditions, the added dynamic forces could, however, become quite considerable as a consequence of the pitching and rolling motions of the boat and they could reach such a magnitude that a considerably larger gas accumulator will be required. After a retardation process, such larger accumulators return a considerable amount of oil to the hydraulic cylinder involved, and the result is that the crane arm will perform a heavy return stroke. Since this return stroke will occur suddenly and without control, there is a great danger for injury to personnel and damage to material and equipment.